A typical steam turbine has a turbine rotor extending horizontally within a turbine casing. The turbine rotor and the turbine casing have a steam channel therebetween. The steam channel is provided with a plurality of turbine stages. Each turbine stage is equipped with a stator blade (turbine nozzle) and a rotor blade (turbine bucket) fitted to the turbine rotor.
Regarding turbine rotor blades used in such a steam turbine, the blade heads often adopts a blade array structure in order to suppress vibration generated during operation or to prevent the steam from leaking through the blade heads.
A blade array structure is formed by joining a plurality of blades to one another to form a single unit. Specifically, these multiple blades are joined to one another by mounting covers onto tenons provided at the blade heads and then caulking the tenons.
In a blade array structure, multiple blades are joined to one another to form a unit, and a certain number of units are provided at the top of turbine rotor blades. However, in addition to time consuming due to a large amount of time required for the caulking process of tenons, such a blade array structure does not necessarily have enough strength at the joint sections. There is known another type of a blade array structure in which all of the blades are joined to one another with covers (integral covers) using a different technique. This type of a blade array structure is known as a full-circumference single-unit blade-array structure.
With regard to a full-circumference single-unit blade-array structure in which the blades are joined to one another with covers, there have provided many technologies which are based on studies on the optimal shape of the covers and the strength and positioning of the joints between the blades and the covers.
FIG. 16 shows an example of turbine rotor blades having a full-circumference single-unit blade-array structure in which an array of blades are joined to each other with covers. Specifically, covers 31, 31 are attached to the top of blades 30, 30. Each of the covers 31, 31 is equipped with bulging sections 34 and 35 that extend from a dorsal blade section 32 side and a ventral blade section 33 side in a circumferential direction 37 of a turbine rotor and in a direction opposite thereto, respectively. The bulging sections 34 and 35 of the neighboring blades 30, 30 are brought into tight contact with each other at their cover contact surfaces 38 extending crosswise to a cover-contact-surface normal line direction (axial direction of the turbine rotor) 36. Under the strong contact force, a reaction force is generated, which is used as a frictional force for suppressing vibration. In other words, a so-called snubber cover structure is disclosed, for example, in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 10-103003).
With a snubber cover structure, a frictional force is produced between the cover contact surfaces 38 of the neighboring blades 30, 30, even if the wheel (i.e. a disk provided on the turbine rotor by integral cutting) undergoes thermal expansion in the radial direction thereof due to a centrifugal force generated during operation or there is an increase in the pitch of the covers 31, 31 caused by a difference in thermal expansion between the wheel and the covers 31. Thus, the positional relationship (face-to-face distance) between the covers 31, 31 is hardly affected by such thermal expansion or an increase in the pitch. Consequently, the positions of the turbine stages used are not subject to limitation even if there are variations in the blade length, there are temperature differences among various positions, or there are differences in linear expansion among the materials used. This allows the free selection of optional turbine stages.
Accordingly, such a snubber cover structure applicable to any of the positions of the turbine stages has been applied to more and more steam turbines in recent years as actual devices.
Although the snubber cover structure disclosed in the Patent Document 1 is advantageous in terms of having the ability to exhibit a high damping effect without having any limitations with respect to the variations in the blade length and the differences in thermal expansion among the materials used, the snubber cover structure still has some problems including a problem related to an assembly process.
Specifically, regarding turbine rotor blades having a snubber cover structure, an assembling process is performed by bringing the cover contact surfaces 38, which are defined by sides of the bulging sections 34 and 35 that are parallel to the circumferential direction 37 of the turbine rotor, into pressure contact with each other when the neighboring covers are brought into contact with each other. Therefore, the dimensions are preliminarily adjusted or the covers are intentionally deformed by means of caulking so as to allow the bulging sections 34 and 35 respectively at the dorsal blade section 32 side and the ventral blade section 33 side to cause interference therebetween.
In these processes performed with respect to turbine rotor blades of this type, the shoulders of the bulging sections 34 and 35 serving as the cover contact surfaces 38 are simply pressed tightly against each other, whereas other contact surfaces are not considered in terms of design. Therefore, even though the shoulders favorably become twisted as a result of reaction forces generated by tightly pressing the shoulders against each other, the twisting is cancelled by the centrifugal force produced during operation. Thus, the generated reaction forces weaken, resulting in the inability to utilize the frictional force, providing a problem of the damping effect being not maintained at a high level.